Flexible linking piece for stabilising the spine

ABSTRACT

The invention relates to a connecting member for maintaining the spacing between at least two anchor members screwed into vertebrae. It comprises two rigid rod-forming parts ( 12, 14 ) made of a first material and each having a fixing, first portion ( 16, 18 ) adapted to be fixed into an anchor member and a fastening, second portion ( 20, 22 ), said rods ( 12, 14 ) being aligned with each other and said fastening portions ( 20, 22 ) facing each other, and a connecting body ( 24 ) made of a second material which is more elastically deformable than said first material and which interconnects said rigid parts ( 12, 14 ) by means of the facing fastening portions ( 20, 22 ) so that said connecting body ( 24 ) is able to deform elastically, whereby the vertebrae, which are held spaced from each other, are movable relative to each other.

[0001] The present invention relates to a connecting member formaintaining the spacing between at least two anchor members which areinterconnected by said connecting member.

[0002] Fields of application of the invention include stabilization andarthrodesis of segments of the vertebral column in degenerativepathologies of the spine.

[0003] Systems for stabilizing the vertebral column by bracing at leasttwo consecutive vertebrae by means of anchor members fixed into saidvertebrae and connected by rigid connecting rods are well known in theart. Systems of this kind are generally coupled systems such that twoconsecutive vertebrae are interconnected by two substantially parallelrods fixed one on each side of the spinous processes. The anchor membersare screwed into the posterior portions of the vertebrae and passthrough the pedicles and a substantial portion of the vertebral bodiesand therefore provide a fixed and durable connection.

[0004] The above stabilizing systems are routinely used to consolidateseveral consecutive vertebrae. Thus the vertebrae are interconnected byrigid rods over a substantial length of the vertebral column. Suchassemblies hold the vertebrae correctly relative to each other; however,they considerably stiffen the spine in terms of bending. It has beenshown that a more flexible stabilizing system, which confers greaterrelative mobility on the vertebrae, is beneficial in some pathologies.

[0005] A first object of the present invention is to provide aconnecting member for maintaining the spacing of existing anchor memberswhile at the same time allowing relative mobility of said anchormembers.

[0006] To achieve the above object, a connecting member in accordancewith the invention, adapted to maintain the spacing between at least twoanchor members screwed into vertebrae, comprises at least two rigidrod-forming parts made of a first material and each having a fixing,first portion adapted to be fixed into an anchor member and a fastening,second portion, said rods being aligned with each other and saidfastening portions facing each other, and a connecting body that is madeentirely from a second material that is more elastically deformable thansaid first material and interconnects the facing fastening portions ofsaid rigid parts so that said connecting body is able to deformelastically, whereby the vertebrae, which are held spaced from eachother, are movable relative to each other.

[0007] Thus a feature of the connecting member lies in the way the tworigid parts are fastened together by means of an elastically deformableconnecting body which imparts relative mobility to the rigid parts whenunder stress, with the reaction force to the stress being proportional,within certain limits, to the deformation of the connecting body. As aresult, the connecting member can be bent by stresses in directions thatare not parallel to the axis of the connecting member; it can also bestretched or contracted by opposing forces acting parallel to the axisof the connecting member.

[0008] Consequently, the two anchor members, when at rest, areinterconnected by the connecting member with its fixing portionsfastened to the anchor members, and can be moved relative to each otherby forces proportional to the movement.

[0009] Said rigid parts are preferably mechanically connected togetherby a single connecting body providing the whole of said mechanicalconnection. In this way a single member provides the connection betweenthe rigid parts at the same time as controlling relative movement of therigid parts. Also, in a particular embodiment, said connecting bodyconsists entirely of a single second material to simplify assembly andto impart homogeneous mechanical properties to it.

[0010] The connecting member of the invention advantageously has n rigidparts with n-1 connecting bodies disposed between them along thelongitudinal axis of said member, each rigid part situated between twoconnecting bodies having one fixing, first portion and two fastening,second portions, there being one fastening, second portion at each endof said fixing, first portion, and said fastening, second portions beingconnected respectively to said two connecting bodies, and the rigidparts at the two ends of said member advantageously have respectivesingle fastening, second portions connected to the connecting bodies,whereby said connecting member is adapted to interconnect n anchormembers.

[0011] Thus, by virtue of this feature, the connecting member maintainsthe spacing between all the anchor members that it interconnects, eachof which can be fixed to a respective vertebra, to align them. Eachrigid part is fixed to an anchor member and, between successive anchormembers, there is a connecting body that interconnects the two fasteningportions. Thus a single connecting member stabilizes several vertebrae,which reduces the time to assemble the stabilizing system as a whole andconsequently the operating time. Also, by virtue of this feature, theconnecting member stabilizes several consecutive vertebrae by connectingthem together, while at the same time making them highly flexible andconferring on them a high degree of relative compressibility in thelongitudinal direction.

[0012] In a preferred embodiment of the invention each of said fasteningportions of said rigid parts that said connecting body interconnects hasa fastening wall to which said connecting body is adapted to adhere.Thus no additional fixing member is needed and the adhesive propertiesof the second material to the fastening wall are sufficient to connectthem.

[0013] In one particular embodiment of the invention, said fasteningwall has openings adapted to cooperate with asperities on saidconnecting body to increase the surface area of contact between saidwall and said body.

[0014] Obviously, providing openings in a wall increases the surfacearea of that wall, which increases the contact area between the twomaterials if one of the materials can be molded onto the wall of theother material. The increase in contact area increases the connectingforces between said connecting body and said fastening portions. Also,the static friction forces of the material of the connecting member onsaid two members are increased in a corresponding manner and theseforces are added to the connecting forces.

[0015] Said second material of which said connecting body is made isadvantageously obtained by polymerization. In this way, the connectingbody can easily be hot molded onto the fastening walls if the materialis polymerized beforehand, or it can be formed in situ if the rate ofpolymerization of the monomers constituting said second material issufficiently low to provide the time necessary for completing theassembly.

[0016] In a preferred embodiment of the invention said first material ofwhich said rigid parts are made is a titanium alloy. It is thereforeeasy to form openings in said fastening wall to which said connectingbody is able to adhere.

[0017] In another preferred embodiment of the invention, the section ofsaid rigid rod-forming parts is circular, which facilitates themanufacture of the member. Also, if prior art circular sectionconnecting rods are to be replaced by connecting rods of the inventionwithout making it necessary to replace the anchor members, it isnecessary for said rigid parts to have sections identical to thesections of the prior art connecting rods.

[0018] The present invention also provides a vertebral stabilizationsystem for fastening together at least two vertebrae each having amedian plane substantially perpendicular to the axis of the spine ofwhich they form a part and a posterior wall defining a posterior medianplane of said spine, said system comprising at least two anchor memberseach adapted to be fixed into the posterior wall of a respectivevertebra so that a line which intersects said two anchor members issubstantially parallel to said axis of the spine, which system furthercomprises at least one connecting member of the invention whose tworigid parts are adapted to interconnect said two anchor members so thatthe axis of said connecting member is substantially parallel to saidaxis of the spine, whereby said vertebrae, which are interconnected viatheir posterior portions, present relative mobility along said axis ofsaid spine.

[0019] Other features and advantages of the invention will emerge onreading the following description of particular embodiments of theinvention, which is given by way of non-limiting example and withreference to the accompanying drawings, in which:

[0020]FIG. 1 is a diagrammatic perspective view of a connecting memberin accordance with the invention,

[0021]FIG. 2 is a diagrammatic view in axial section of the connectingmember in accordance with the invention,

[0022]FIG. 3 is a diagrammatic perspective view showing anchor membersconnected by the connecting member, and

[0023]FIG. 4 is a diagrammatic side elevation view of a vertebral columnshowing two consecutive vertebrae into which there are screwed anchormembers interconnected by a connecting member in accordance with theinvention.

[0024] The various portions of a connecting member of the invention aredescribed initially with reference to FIG. 1.

[0025] The connecting member 10 has two cylindrical rigid parts 12 and14. Each rigid part 12, 14 has a fixing, first portion 16, 18 and afastening, second portion 20, 22 forming an enlargement. The facingfastening portions 20 and 22 are connected together by a connecting body24 so that the rigid parts 12 and 14 are in axial alignment. Theconnecting member 10 is therefore circularly symmetrical about the axisA.

[0026] How the two rigid parts 12 and 14 are fastened together isdescribed below with reference to FIG. 2.

[0027] The connecting body 24 is a plastics material body obtained bypolymerization. The material of the body is chosen from materials whichare more elastically deformable than the material of said rigid parts12, 14 and, most importantly, whose elastic properties are of the sameorder of magnitude as those of the posterior ligaments that hold thespine together.

[0028] Organic silicon compounds constitute polymers whose mechanicalproperties can be determined by the choice of their basic components, inparticular by their degree of substitution, the nature of thesubstituents, and their molecular weight, and whose elastic behaviorpredominates over its plastic behavior. They therefore constitute afamily of materials suitable for interconnecting the two rigid parts 12and 14. Also, these polymers can adhere strongly to materials ofinorganic composition. Thus the connecting body 24 provides good meansfor fastening together the rigid parts 12, 14, which are generally madeof titanium alloy.

[0029] Nevertheless, the polymer materials that can be used are notlimited to organic silicon compounds, and any other material havingcomparable properties could be suitable.

[0030] The material of the connecting body 24 is adapted to adhere tothe fastening walls 20′ and 22′ of said fastening second portions 20,22. However, to increase the adhesion, openings 30, 32 are formed in thefastening walls 20, 22 of the fastening, second portions and are adaptedto cooperate with asperities 26, 28 on the connecting body 24 which areinserted into the openings 30, 32.

[0031] This feature increases the contact area between the two materialsand thereby increases the connecting force between them in a directionnormal to said surface of contact and creates static friction forceswhich are additional to the adhesion force.

[0032] A connection of the above kind is obtained either by injectingthe polymer while hot between the two rigid parts 12 and 14 held facingeach other in a mold, or by cold molding the mixture of monomers betweenthe two rigid parts 12 and 14, if the speed of the reaction issufficiently low. The asperities 26, 28 are therefore formed in situ,when the polymer liquid or paste inserted into the openings 26, 28solidifies after cooling or after a chemical reaction. Obviously, theconnecting body 24 consists of the polymer disposed between the rigidparts 12 and 14, more specifically between the fastening walls 20′ and22′, and, in order to retain the polymer between the facing portionswhile it is in the liquid state, the walls of the mold must necessarilysurround the space between and in line with the two rigid parts 12, 14.

[0033] In a particular embodiment (not shown) the openings 30, 32 formedin the fastening walls 20′ and 22′ open onto the outside wall of therigid parts 12 and 14 so that the liquid polymer penetrates entirelyinto the openings 30, 32 without it being possible for air to be trappedtherein. This reinforces the fastening of the connecting body 24 to therigid parts 12, 14.

[0034] Also, the openings 30, 32, which are shown as parallel to thelongitudinal axis of the connecting member in FIG. 2, can be oblique tothat longitudinal axis and/or not rectilinear. These configurationsincrease the static friction forces of the polymer on the rigid parts,which fastens them together more strongly.

[0035] Now that the manner in which the two rigid parts are fastenedtogether has been described, movement of the rigid parts relative toeach other is described with reference to FIG. 1.

[0036] Given the circular symmetry of the rigid parts 12 and 14 and theconnecting body 24, and the nature of the material of the connectingbody 24, the connecting member 10 is able to bend in all directions in aplane Pp perpendicular to the axis A of the connecting member when thetwo first portions are immobilized. Bending of the connecting member 10compresses one edge of the connecting body 24 and stretches thediametrally opposite edge, whereas the rigid parts 12 and 14 retaintheir shape. Because the material of the connecting body 24 iselastically deformable, when the stresses causing the bending areremoved, the connecting member 10 returns to its original state in whichthe rigid parts 12 and 14 are in axial alignment.

[0037] Also, the rigid parts 12 and 14 can move relative to each otherin opposite directions along the longitudinal axis A to compress orstretch the connecting body 24.

[0038] The relative movement of the two rigid parts 12 and 14 can occurin directions other than the directions described above, but theconnecting member is principally loaded in bending, tension andcompression, as described in more detail below.

[0039] Deformation of the connecting member connected with relativemovement of the anchor members 42 and 44 is described next withreference to FIG. 3.

[0040]FIG. 3 shows the connecting member 10 whose two rigid parts 12 and14 interconnect the two anchor members 42 and 44. The two anchor members42 and 44 are parallel to each other in a common axial plane Pa.

[0041] Each anchor member 42, 44 has a threaded shank 46 with a U-shapedhead 48 at the top whose inside wall is threaded so that a screw-formingmember 50 can be screwed into it. Thus the first portions 16 and 18 ofthe rigid parts 12 and 14 are accommodated in the heads 48 of therespective anchor members 42 and 44 and are locked to them by tighteningthe screw-forming members 50.

[0042] As a result, when the threaded shanks 46 of the anchor membersmove towards each other due to the effect of opposite forces T and −T inthe plane Pa and substantially parallel to the axis A the anchor members42 and 44 deform the connecting member, which bends.

[0043] The bending of the connecting member 10 compresses the lower edge52 of the connecting body 24 and stretches the diametrally oppositeupper edge 54, while the rigid parts 12 and 14 retain their shape.Because the material of the connecting body 24 is elasticallydeformable, when the stress is removed the connecting member reverts toits original rectilinear shape and the threaded shanks of the anchormembers 46 return to their former relative position.

[0044] The mechanism of elastic bending of the connecting member 10 andthe anchor members 42, 44 described above is the same if the threadedshanks 46 of the anchor members 42 and 44 move away from each other, theconnecting member bending with the opposite curvature.

[0045] Also, the anchor members 42 and 44 are movable in translationrelative to each other along the axis A, their relative movementstretching or compressing the connecting body 24.

[0046] The use of the connecting member 10 in a vertebral stabilizationsystem for fastening together at least two vertebrae V1 and V2 isdescribed below with reference to FIG. 4.

[0047] The vertebrae V1, V2 each have respective median planes PV1, PV2substantially perpendicular to the axis Ar of the spine of which theyform part, and respective posterior walls PPV1, PPV2 defining aposterior median plane PPr of said spine.

[0048] The stabilizing system includes at least two anchor members 42and 44 respectively screwed into the posterior walls PPV1, PPV2 of thevertebrae V1, V2, so that a line L that intersects the two anchormembers 42 and 44 is substantially parallel to said axis Ar of thespine. The two first portions 16 and 18 of the connecting member 10interconnect the two anchor members 42 and 44. As a result, thevertebrae V1 and V2, which are interconnected in their posteriorportions, possess relative mobility along the axis Ar of the spine.

[0049] Thus when the spine is stretched, the vertebrae V1 and V2 moveaway from each other in opposite directions E and −E, which causes thethreaded shanks 46 to move away from each other, deforming theconnecting member 10, and in particular its connecting body 24. This isbecause the connecting body is compressed both longitudinally and at theupper edge 54. The deformed connecting member has it concave side facingaway from the spine.

[0050] When the spine is bent, the inverse effect occurs and thevertebrae V1 and V2 move towards each other, which induces deformationof the connecting member with its concave side facing toward the spine.

[0051] The connecting body is then subjected to longitudinal extensionof its upper edge 54 and possibly to compression of its lower edge 52.

[0052] It will be understood that the connecting member 10 in accordancewith the invention achieves greater relative mobility of the vertebraecompared to the prior art connecting rods, which cannot be compressedlongitudinally.

[0053] In a particular embodiment (not shown) the connecting member hasthree rigid rod-forming parts and two connecting bodies interconnectingthe three rigid parts. To this end, the central rigid part includes afixing, first portion and two fastening, second portions, with onefastening, second portion on each side of said fixing, first portion,said fastening, second portions being connected to said two connectingbodies. The other two rigid parts, situated at the two ends of saidconnecting member, have a single fastening, second portion connected tothe connecting bodies.

[0054] The connecting member therefore maintains the spacing betweenthree anchor members that it interconnects, which are fixed to threesubstantially equidistant vertebrae, to align them. Each rigid part ofthe connecting member is fixed to an anchor member so that there arerespective elastically deformable connecting bodies between the pairs ofvertebrae. In this way, a single connecting member stabilizes threevertebrae, which reduces the time needed to assemble the stabilizingsystem as a whole and consequently the operating time. Also, because thethree vertebrae are interconnected by a single connecting member, theirmobility relative to each other is better controlled.

[0055] It goes without saying that providing connecting members havingmore than three rigid parts connected together by elastically deformableconnecting bodies would not depart from the scope of the invention.

1. A connecting member for maintaining the spacing between at least twoanchor members screwed into vertebrae, characterized in that itincludes: two rigid rod-forming parts (12, 14) made of a first materialand each having a fixing, first portion (16, 18) adapted to be fixedinto an anchor member and a fastening, second portion (20, 22), saidrods (12, 14) being aligned with each other and said fastening portions(20, 22) facing each other, and a connecting body (24) that is madeentirely from a second material that is more elastically deformable thansaid first material and interconnects the facing fastening portions (20,22) of said rigid parts (12, 14) so that said connecting body (24) isable to deform elastically, whereby the vertebrae, which are held spacedfrom each other, are movable relative to each other.
 2. A connectingmember according to claim 1, characterized in that said rigid parts aremechanically connected together by a single connecting body providingthe whole of said mechanical connection.
 3. A connecting memberaccording to claim 1 or claim 2, characterized in that said connectingbody is entirely made of a second material.
 4. A connecting memberaccording to any one of claims 1 to 3, characterized in that it includesn rigid parts with n-1 connecting bodies disposed between them along thelongitudinal axis of said member, each rigid part situated between twoconnecting bodies having one fixing, first portion and two fastening,second portions, there being one fastening, second portion at each endof said fixing, first portion, and said fastening, second portions beingconnected respectively to said two connecting bodies, and in that therigid parts at the two ends of said member have a single fastening,second portion connected to the connecting bodies, whereby saidconnecting member is adapted to connect n anchor members.
 5. Aconnecting member according to any one of claims 1 to 4, characterizedin that said fastening portions (20, 22) of said rigid parts (12, 14)connected by said connecting body (24) have a fastening wall (20′, 22′)to which said connecting body (24) is adapted to adhere.
 6. A connectingmember according to claim 5, characterized in that said fastening wall(20′, 22′) has openings (30, 32) adapted to cooperate with asperities(26, 28) on said connecting body (24) to increase the contact areabetween said wall (20′, 22′) and said connecting body (24).
 7. Aconnecting member according to any one of claims 1 to 6, characterizedin that said second material of which said connecting body (24) is madeis obtained by polymerization.
 8. A connecting member according to anyone of claims 1 to 7, characterized in that said first material of whichsaid rigid parts (12, 14) are made is a titanium alloy.
 9. A connectingmember according to any one of claims 1 to 8, characterized in that thesection of said rigid rod-forming parts (12, 14) is circular.
 10. Avertebral stabilization system for fastening together at least twovertebrae (V1, V2) each having a median plane (PV1, PV2) substantiallyperpendicular to the axis (Ar) of the spine of which they are part and aposterior wall (PPV1, PPV2) defining a posterior median plane (PPr) ofsaid spine, said system comprising at least two anchor members (42, 44)each adapted to be fixed to the posterior wall (PPV1, PPV2) of avertebra so that a line which intersects said two anchor members issubstantially parallel to said axis (Ar) of the spine, the system beingcharacterized in that it further comprises at least one connectingmember (10) according to any one of claims 1 to 9 whose two rigid parts(12, 14) are adapted to interconnect said two anchor members (42, 44) sothat the axis (A) of said connecting member is substantially parallel tosaid axis (Ar) of the spine, whereby said vertebrae (V1, V2), which areinterconnected via their posterior portions, present relative mobilityalong said axis (Ar) of said spine.